1. Field of the Invention
The present invention relates to a device for testing integrated circuits and, more particularly, to a device for testing integrated circuits in semiconductor wafer form at temperatures elevated or reduced from room temperature.
2. Description of the Art
In the manufacture of integrated electronic semiconductor circuits, especially large-scale microelectronic circuits, a complete circuit can contain thousands of individual transistor elements but may be less than one-quarter inch on a side. In practice, a hundred or more of such circuits can be simultaneously fabricated, side by side, on a single wafer of pure, single-crystal silicon. Typically, the silicon wafers are about twenty five thousandths of an inch in thickness and about three to four inches in diameter; however, efforts are being made to utilize even larger diameter wafers.
After the microelectronic circuits are fabricated together on a single wafer, it is conventional practice to place the wafer in a testing machine to individually test the circuits (known as "dies") before the wafer is broken into individual die, each containing a complete circuit. It is well known to accomplish such testing by use of a probing device carrying an array of fine needle-like probes which make electrical contact with the contact pads on the individual dies. The probing device is normally mounted on a computer-controlled testing machine which holds the wafer in a chuck and sequentially positions chuck relative to the the probing device so that the individual dies can be tested by establishing contact between the probes and the circuit pads of the die to be tested. Then a testing device, usually called an autotester, electrically tests the integrated circuit. If the circuit is defective, the die is marked with an ink spot or its position is otherwise recorded to indicate that the die should be discarded. The testing machine then steps to the next die and repeats the operation. Such a machine is shown, for example, in "Scientific American", Sept. 1977, page 120. Today, such machines can test a die in about 250 milliseconds (i.e., one-quarter second). When tested in such a manner as part of a complete wafer, the dies are said to be tested in wafer form.
Workers in the art have attempted to utilize such wafer-testing machines to test integrated circuits in wafer form under conditions of elevated or reduced temperature as well as ambient conditions (normal room temperature). Such testing over a range of temperatures is important because operational characteristics of integrated circuits can vary substantially with temperature, and, in some situations, an integrated circuit which is operational at room temperature may become inoperable at relatively high or low temperatures. Especially for military and aerospace applications, it is often desirable to test integrated circuits at temperatures ranging from -55.degree. C. (-67.degree. F.) to +125.degree. C. (257.degree. F.) with an accuracy of .+-.1.degree. C. To accomplish such testing, workers in the art have heated or cooled the chuck which holds the wafer to bring the circuits to the desired temperature, and then tested each of the integrated circuit dies on the wafer at the elevated or reduced temperature. The procedure of utilizing the chuck to elevate or depress the temperature of a complete wafer is relatively time-consuming and can cause condensation which may result in erroneous test results. In addition, thermal gradients may exist across the chuck or wafer which interfere with the accuracy of the test results. Such effects become more acute as the temperature of testing is increased or decreased further from room temperature.